Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. The anode and cathode each comprise electrocatalyst, which is typically disposed at the membrane/electrode layer interface, to induce the desired electrochemical reaction.
At the anode, the fuel moves through the porous anode layer and is oxidized at the electrocatalyst to produce protons and electrons. The protons migrate through the ion exchange membrane towards the cathode. On the other side of the membrane, the oxidant moves through the porous cathode and reacts with the protons at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
Electrochemical fuel cells can operate using various reactants. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
The fuel stream may contain impurities which do not contribute to, and may actually inhibit, the desired electrochemical reaction. These impurities may, for example, originate from the fuel stream supply itself, or may be generated in situ in the fuel cell, for example, as intermediate species during the fuel cell reactions. Further, impurities may enter the fuel stream from elsewhere in the system. Some of these impurities may be chemically adsorbed or physically deposited on the surface of the anode electrocatalyst, blocking the active electrocatalyst sites and preventing these portions of the anode electrocatalyst from inducing the desired electrochemical fuel oxidation reaction. Such impurities are known as electrocatalyst "poisons" and their effect on electrochemical fuel cells is known as "electrocatalyst poisoning". Electrocatalyst poisoning thus results in reduced fuel cell performance, where fuel cell performance is defined as the voltage output from the cell for a given current density. Higher performance is associated with higher voltage for a given current density or higher current for a given voltage.
In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. However, known pre-treatment methods for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 ppm can eventually result in electrocatalyst poisoning which causes a reduction in fuel cell performance.
Substances other than carbon monoxide are also known to poison fuel cell electrocatalysts. Depending on the type of fuel and the fuel processing methods, impurities in the fuel stream may be present in quantities sufficient to poison the electrocatalyst and reduce fuel cell performance. Fuel cell components and other fluid streams in the fuel cell system may also be a source of impurities which may result in poisoning of the electrocatalyst. For example, fuel cell separator plates are commonly made from graphite. Organic impurities in the graphite may leach out and poison the electrocatalyst. Other poisons may be generated by the reaction of substances in the reactant streams with the fuel cell component materials.
What constitutes a poison may depend on the nature of the fuel cell. For example, whereas methanol is the fuel in a direct methanol fuel cell, in a hydrogen fuel cell operating on a methanol reformate stream, traces of unreformed methanol can be detrimental to the electrocatalyst performance.
Conventional methods for addressing the problem of anode electrocatalyst poisoning include purging the anode with an inert gas such as nitrogen. However, such purging methods involve suspending the generation of power by the fuel cell, thus a secondary power source is needed to provide power while the fuel cell anode is being purged.
Another approach for removing poisons from an electrocatalyst comprises introducing a "clean" fuel stream containing substantially no carbon monoxide (or other poisons) to a poisoned fuel cell anode. Where the adsorption is reversible, an equilibrium process results in some rejuvenation of the electrocatalyst. However, a disadvantage of this approach is that it is generally not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the anode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is not able to operate at full capacity.
Another approach to counteract carbon monoxide electrocatalyst poisoning is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed by Gottesfeld in U.S. Pat. No. 4,910,099. However, there are several disadvantages to Gottesfeld's method which influence fuel cell performance and efficiency. For example, an oxygen bleed results in parasitic losses, undesirable localized exothermic reactions at the anode, and dilution of the fuel stream.
It is apparent from the prior art that there is a need for an improved method and apparatus for rejuvenating a fuel cell anode electrocatalyst by removing poisons therefrom, which does not involve suspending the availability of the fuel cell to generate power.